This invention relates to indirect calorimeters for determining the metabolic rates of subjects by measuring their oxygen consumption during respiration over a period of time, and more particularly to such a calorimeter employing the utility of a flow meter and a component gas concentration sensing device.
Measurement of the energy expenditure of humans is important for a number of reasons, including the determination of the proper caloric content for feedings of hospitalized patients whose metabolisms may deviate from normal values, the monitoring of progress of weight loss diets to allow the adjustment of caloric inputs to achieve a target loss and the determination of energy expenditure during exercise.
A variety of indirect calorimeters for measuring oxygen consumption during respiration have been devised. One form of respiratory calorimeter, disclosed in U.S. Pat. Nos. 4,917,108; 5,038,792; 5,179,985 and 5,178,155, all to Mault, measures the volume of a subject""s inhalations over a period of time, and the volume of the subject""s exhalations after carbon dioxide in the exhalations has been removed by an absorbent scrubber. These measurements are integrated over the time of measurement and the difference between the two summed volumes is a measure of the subject""s oxygen consumption. This follows from the fact that inhaled oxygen is either absorbed into the blood in the subject""s lungs or expelled during exhalation. Some portion of the blood absorbed oxygen is replaced with CO2. When the CO2 is removed from the exhaled volume, the summed difference between inhalation and exhalation volume over a period of time is equal to the absorbed oxygen.
In some versions of these prior calorimeters, a capnometer was also used to measure the instantaneous value of the exhaled CO2 in a breath allowing the calculation of CO2 production, Resting Energy Expenditure (REE) and Respiratory Quotient (RQ).
An absorbent scrubber used with these previous systems, such as sodium hydroxide or calcium hydroxide, which reacts with the CO2 to form water plus a salt, has a limited ability to absorb CO2 and must be replenished after a period of use. The scrubber is also large and heavy relative to the other components of the calorimeter. Thus, there is a need in the art for an indirect calorimeter that utilizes a component gas concentration sensor in place of a scrubber to determine the volume of exhaled carbon dioxide.
The present invention is an indirect calorimeter operative to measure the respiratory oxygen consumption per unit time of a subject. The indirect calorimeter includes a respiratory connector operative to be supported in contact with a subject, so as to pass respiratory gases as the subject breathes into the respiratory connector and a bi-directional flow meter having an ultrasonic flow transducer that bi-directionally transmits and receives ultrasonic signals to the transducer to generate a signal as a function of the volume of gases passing through the flow meter. The indirect calorimeter also includes a gas concentration sensor operative to generate a signal as a function of an instantaneous carbon dioxide content of gases passing by the gas concentration sensor. The indirect calorimeter further includes a computer operative to receive signals from the flow meter and the gas concentration sensor, and conduits interconnecting the respiratory connector, the flow meter and the gas concentration sensor, so that the subject""s inhalations and exhalations pass through the flow meter and the subject""s exhalations pass over the gas concentration sensor and the computer is operative to receive the signals from the gas concentration sensor and the flow meter to calculate the subject""s oxygen consumption.
One advantage of the present invention is that the need for the carbon dioxide scrubber is eliminated by measuring the volume of exhaled carbon dioxide and subtracting that volume from the total exhaled volume over the measurement period to calculate a summed difference that is then subtracted from the inhaled volume to arrive at VO2 consumed. Another advantage of the present invention is that the volume of exhaled carbon dioxide is determined by integrating the instantaneous carbon dioxide percentage of the exhalation, as measured by a component gas concentration sensing device, over the exhaled volume as measured by a flow meter: VCO2=Ve(% CO2).
The flow meter generates an electrical signal as a function of the instantaneous flow volume and this signal is preferably sent to a microprocessor-based computer along with the electrical output of a capnometer sensor. Still another advantage of the present invention is that the gas concentration sensing device generates electrical output signals representative of certain gaseous percentages, namely O2 and/or CO2, in the inhaled and exhaled flow volume and these signals are sent to the microprocessor-based computer along with the signal from the flow meter. Still yet another advantage of the present invention is that a bi-directional flow meter is used to measure both the inhaled and exhaled flow volume. A temperature and/or humidity conditioner may be utilized to equalize the temperature and/or humidity of the incoming air to that of the exhaled air, so that uniform flow measurements may be made. Alternatively, the system could receive signals representing temperature, humidity and/or barometric pressure from sensors disposed in the calorimeter or externally, or keyboard entries and calculate correction factors for the flow measurement based on the signals. A further advantage of the present invention is that the distinction between inhalations and exhalations is determined by the presence or absence of CO2 in the flowing gas as measured by the component gas concentration sensing device, such as an infrared absorption sensor, that determines the amount of a particular gas in a volume of inhaled or exhaled air and outputs an electrical signal representative of the measured amount.
Yet a further advantage of the present invention is that the microprocessor, in addition to calculating and displaying the VO2, is used to calculate and display REE, RQ and the rate of carbon dioxide production.
Still yet a further advantage of the present invention is that the subject""s Cardiac Output is calculated using a noninvasive method of cardiac output measurement using partial CO2 rebreathing described in an article by Capek and Roy in IEEE Transactions and Biomedical Engineering, Vol. 35, pages 653-61, 1988. This embodiment of the invention employs a two stage measurement. In the first stage, the device is configured in essentially the same manner as the other embodiments of the invention to measure oxygen consumption. Over a period of use, such as three minutes, the microprocessor measures VO2, VCO2, and the end-tidal CO2 (etCO2) which is the carbon dioxide content of a breath at the end of an exhalation. These values are stored and the device is then switched to a configuration in which the end portion of each exhalation is not expelled from the device, but is rather captured, so that it forms the initial portion of the gas provided to the subject during the next inhalation. This is achieved by creating a dead space chamber in the exhalation passage. The subject breathes in this manner for a short period such as 30 seconds. During this period the breath-to-breath etCO2 and the total VCO2 are recorded. The computer then implements the calculation:       C    .    O    .    =            Δ      ⁢              xe2x80x83            ⁢              VCO        2                    Δ      ⁢              xe2x80x83            ⁢              etCO        2            
where xcex94VCO2 equals the difference in the total volume of exhaled CO2, per breath, during the two recordings and xcex94etCO2 is the difference in the end-tidal CO2 between the two recordings.
Other features and advantages of the present invention will be readily appreciated, as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings.